There is a fast growing need for accurate measurement of pressures and differential pressures of pure gases as well as mixtures of gases in vacuum systems used in the manufacture of computer chips and similar high technology devices. So-called capacitance manometers because of their cleanliness and sensitivity are widely used for this purpose almost to the exclusion of all other types of devices.
In a capacitance manometer, a relatively thin diaphragm separates the gas in pressure chamber 1 from the gas in chamber 2. A pressure differential across the diaphragm causes it to be deflected until the deformation induced force is equal to the pressure induced force. A capacitance probe is used to determine the amount of deflection which may then be related to the pressure difference across the diaphragm.
Capacitance manometers suffer from a number of deficiencies. For example:
1. In order to measure small differential pressures, the diaphragm must be thin and flat so that the spring rate is very small. However, a thin flat diaphragm is easily damaged by excessive stresses caused by over pressure conditions. Various diaphragm support means are known but costly over pressure accidents still occur. Because the flat diaphragm deforms in a complex shape, effective stops are also relatively expensive to machine. PA1 2. Because of the use of a thin flat diaphragm, capacitance manometers have limited pressure range, typically only four decades. Therefore, multiple manometer heads must be installed to provide pressure measurement over the 7 or 8 decades of pressure range typically required. PA1 3. Because the diaphragm is purposely deformed during the measuring process, capacitance manometers are extremely sensitive to changes in material properties with temperature or age. PA1 4. Capacitance manometers have sensitive electrical circuits exposed on one side of the diaphragm which are readily damaged by process gases. PA1 5. Capacitance manometers require costly vacuum compatible electrical feedthroughs to conduct electrical signals through the vacuum wall. PA1 6. Capacitance manometers suffer from hysteresis effects. The pressure indication at a given pressure depends on whether the given pressure is approached by increasing or decreasing the pressure. PA1 7. Capacitance manometers require extreme care in manufacture to assure the thin diaphragm is flat and uniformly stressed and are therefore very costly to manufacture. PA1 1. Has a very wide differential pressure range suitable for clean vacuum processing. PA1 2. Is relatively immune to over pressure. PA1 3. Has negligible hysteresis. PA1 4. Is relatively insensitive to the effects of changes in material properties with temperature and age. PA1 5. Has position sensing and restoring force means located external to the vacuum. PA1 6. Has no electrical circuits inside the vacuum and therefore no need for vacuum feedthroughs.
Clearly what is required is a manometer which has a wide range, and is immune to overpressure, hysteresis and changes in material properties.